As a recent progress in mobile communication technology, 3G (3rd Generation: third generation mobile phones) standards have been studied by the 3GPP/3GPP2 (3rd Generation Partnership Project), which is organized by carries from various countries centered around the International Telecommunication Union (ITU). The 3G standards have been studied as IMT-2000 (International Mobile Telecommunication-2000), support transmission of various data such as audio, texts, still pictures, and motion pictures, which require different transmission rates, provide high speed transmission as well, and are intended for connection to the Internet and the like. There have already been released plural types of systems such as W-CDMA for Japan and Europe, and cdma2000 for the United States.
In these CDMA systems, respective channels of respective mobile station (also referred to as wireless terminal or subscriber terminal, referred to as MS hereinafter) and BTS (Base Transceiver Station, referred to as BTS hereinafter) are interfered by radio waves output by MS's and BTS's other than the MS and BTS itself, resulting in degradation of wireless line quality (simply referred to as quality hereinafter). This quality is also influenced by various factors such as the distance between the BTS and MS, the multipath environment, and the travel speed. Transmission power control (referred to as TPC hereinafter) is used to prevent the channel interference, to maintain the proper quality, and to increase the throughput of the overall system, thereby increasing the number of subscribers.
This TPC implies that the transmission powers of the MS itself and BTS itself are controlled to proper values based on control information (command) on the transmission powers contained in a frame transmitted from a BTS of opposite party (opponent BTS) and an MS of opposite party (opponent MS). Note that the opponent BTS and opponent MS are referred to as opponent apparatus (associated equipment) in the following description.
As the TPC, there is known inner loop control. The inner loop control implies control where the MS inserts TPC bits as the control information to an upstream slot, and the BTS then increases/decreases the transmission power values of the BTS and MS based on the TPC bits.
FIG. 20 describes the inner loop control and outer loop control. The inner loop control L1 shown in FIG. 20 controls the transmission powers of both the BTS and MS, and the BTS applies feedback control to the transmission power from the BTS itself, and the transmission power of a signal from the MS based on a receive power value of the signal transmitted by the MS. Namely, the BTS stores a receive power threshold value for the MS in advance, and transmits a command to the MS to increase the transmission power of the MS if the receive power value from the MS is lower than the receive power threshold value. Alternatively, the BTS transmits the TPC information to the MS to decrease the transmission power of the MS if the receive power value from the MS is higher than the receive power threshold. SIR (Signal Interference Ratio: SI ratio) is used as the receive power value, for example.
The description above is given of the inner loop control.
On the other hand, the outer loop control L2 shown in FIG. 20 is used for a base station controller (referred to as BSC hereinafter) to set the receive power threshold value of the inner loop control L1 by the BTS. The BSC estimates an average frame error rate (referred to as FER hereinafter, and also implies FER value unless otherwise specified) of the frames received by the BTS, and controls the receive power threshold value such that the FER remains at a desired value. As a result, proper TPC is provided according to changes in the transmission environment.
Note that open loop control is used for a mobile communication system to which the inner loop control cannot be applied such as a random access channel. The open loop control is carried out such that an MS estimates a transmission loss based on a receive power value of a signal transmitted by a BTS on the MS, thereby controlling the transmission power value of the MS itself.
A description will now be given of a calculation example of the FER of the outer loop control.
The target value of the quality depends on a service to be provided. For example, the FER for audio communication is 1.0%, and the FER for data communication is 0.1% or 0.2%. If a receiving section of the BTS or BSC processes the audio communication, the outer loop control is carried out such that the FER achieves 1.0%, and the FER is acquired using an equation (Z1).FER=(Number of frames of CRCNG or number of irregular frames such as corrupt frames)/Number of entire received frames  (Z1)
The CRC (Cyclic Redundancy Check) is used to detect burst errors which occur in succession, and CRCNG (CRC No Good), which implies that a burst error is detected, or CRCOK (CRC O.K.), which implies that the number of errors is within a permissible range, is output by the receiving section, for example. “Corrupt” implies a frame which includes a large number of errors, and thus is not recognized as a frame. Note that “/” represents division.
There are proposed a large number of technologies relating to the transmission power control using the outer loop.
Outer loop control, which is described in Technical Report of IEICE, RCS98-18 “Experimental Study of Adaptive Transmission power Control Using Outer Loop in W-CDMA” (referred to as publicly known document 1), determines the CRS for respective wireless frames, measures (actually measures) the FER by means of counting numbers of CRCOK/CRCNG, and then compares the measured FER and a target FER with each other, thereby updating a target receive power value (or a target receive level/interference level ratio). In addition, this publicly known document 1 describes that if an FER observation period (observation number) includes 1000 frames or more, the system can control such that the FER remains approximately at a desired constant value. If there is a sufficiently long FER observation period, the quality can be maintained constant by the outer loop control.
However, according to the publicly known document 1, an observation period long enough for securing the precision of the set value is necessary in order to maintain the quality of the target FER. Specifically, there are following problems, (i) to (iii), relating to the observation period using the conventional method.
(i) A mobile communication system needs a period including at least 100 frames to secure an FER of 1.0%, and a period including more frames to secure a precision corresponding to approximately an FER of 0.1%. Moreover, to secure a precision exactly corresponding to 1.0%, the system needs an observation period including 1000 frames or more as described in the publicly known document 1.
In this case, the period used for estimating the FER increases, and an update cycle of the target receive power value (or target receive power/interference power ratio) thus increases. Accordingly, there increases a period where the inner loop is not operating at a proper value in terms of the receive power value satisfying the target FER, resulting in failing to secure the desired FER.
(ii) If the update cycle is reduced such that the FER achieves the target FER according to the change in the transmission environment, and the target receive power value follows the change of the transmission environment, the FER to be set as the target is restricted by the time period or the number of frames. Accordingly, the desired FER is not achieved.
(iii) In addition, there have recently been increasing demands for high speed data communication. Many protocols for the mobile communication system are designed such that an upper layer assembles data from a lower layer (such as wireless frames). Accordingly, when this type of protocol is used, even if the FER is small on a lower layer, NG frames (error frames) are included when an upper layer assembles the frames, and the frame assembled by the upper layer results in NG frames.
To accommodate this problem, if the BTS or MS receives the NG frame, the upper layer operates to obtain the NG frame again by means of retransmission control. If there often occur retransmitted frames as a result, the throughput used in an actual service decreases on the upper layer compared with the wireless frames actually transmitted/received.
It is thus necessary to reduce the FER (to increase the quality) for providing the high speed data communication service.
Alternatively, Japanese Patent Laid-Open (Kokai) No. HEI 8-181653 (referred to as publicly known document 2 hereinafter) describes a transmission power control method, where either an MS or BTS includes measuring means which measures receive qualities of respective wireless lines, an opponent station includes learning means which learns an average line quality characteristic on the opponent station from the line quality measured by the measuring means, and a target carrier power/interference power ratio is set based on the average line quality characteristic leaned by the learning means. Consequently, the transmission power can be restrained to a necessary minimum value.
However, the transmission power control method described in the publicly known document 2 does not acquire a target receive power value at a high speed.
Thus, when the conventional art is used, if a desired FER value is high, the overall throughput decreases due to the frame retransmission. On the other hand, a long measuring segment (measurement period) is necessary to obtain a FER at a high precision. In addition, if the update cycle of the FER is shorten, an FER at a high precision is not acquired, and thus, a proper power threshold value cannot be set.
On this occasion, it is necessary to reduce the FER in order to reduce the number of the retransmissions. If the FER is small, a considerably long period is necessary to increase the measurement precision of the FER. In this case, the measurement precision of the FER cannot sufficiently follow a transmission characteristic, which changes due to the travel of the mobile station.
The present invention is devised in view of the foregoing problems, and has an object of providing a transmission power control apparatus, a mobile communication system, and a power control method which can carry out the TPC at a high speed, improve the quality of data on a lower layer such as wireless frames, maintain the quality constant, and increase the throughput.